Aluminum alloys are important in many industries. Glassy Al-based alloys and their devitrified derivatives are currently being considered for structural applications in the aerospace industry. These alloys involve the addition of rare earth and transition metal elements. These alloys have high strength and, when processed appropriately, have high ductility.
One of the key requirements for high ductility is control of the second phase size during thermomechanical processing; in this case, extrusion into various extruded shapes.
When aluminum or aluminum alloys are extruded, the alloys, depending on the alloy composition, are heated to between 700° F. (375° C.) and 800° F. (427° C.), and are extruded through shear-faced dies with a high extrusion ratio and at high ram speeds. This functions to impart as much “work” into the alloy as possible. There is no concern for adiabatic heating because the alloys are usually heat-treatable. The alloys can be solutionized, quenched and aged to a desireable temper after extrusion.
Because glassy Al-based alloys have different structures, the temperatures noted above along with adiabatic heating from the shear-faced dies promote almost instantaneous devitrification so that the benefits of the glassy state are lost. Also, derivatives of the glassy state produce nanocrystalline microstructures that have mechanical properties that cannot be obtained when starting out with powder in the crystalline state. Al-based alloys such as Al—Y—Ni—Co alloys are devitrified glass-forming aluminum alloys that derive their strength from a nanometer-sized grain structure and nanometer-sized intermetallic second phase or phases. Examples of such alloys are disclosed in co-owned U.S. Pat. Nos. 6,974,510 and 7,413,621, the disclosures of which are incorporated herein by reference in their entirety. Both devitrified aluminum alloys with nanocrystalline microstructures and those that are glassy without being devitrified have not been successfully extruded using conventional extrusion practices.
A new approach to extrusion of glassy Al-based powder is needed.